In the process of coking coal in the absence of air, large volumes of gas, commonly called coke oven gas, are produced in addition to carbonaceous residue. This coke oven gas contains valuable by-products, such as ammonia, naphthalene, tar and light oils comprising benzene, toluene, xylenes and other hydrocarbons which are typically recovered in by-product recovery systems associated with the coke oven plants.
In the by-product recovery processes currently in use, the hot coke oven gas leaves the coking furnaces at a temperature of 600.degree.-700.degree. C. and is shock cooled by a spray of aqueous flushing liquor in a collecting main. This cooling effects a condensation of some of the vapors and removes heavy tar from the coke oven gas. The non-condensed gases at about 75.degree.-80.degree. C. are directed to a primary cooler where further cooling to about 35.degree.-50.degree. C. by spraying with water or "ammonia liquor" removes additional tar. Part of the ammonia present in the gas is absorbed in the aqueous liquid in the primary cooler and, together with the tars and a large portion of the naphthalene that are condensed, is carried away with the "ammonia liquor". Any tar remaining in the gas is usually removed in a subsequent electrostatic precipitator.
After passing through the primary coolers and electrostatic precipitators, the cooled coke oven gas is contacted with sulfuric acid in an ammonia absorber to remove any remaining ammonia preparatory to treatment of the gas for the recovery of light oils and subsequent scrubbing to eliminate hydrogen sulfide. Although the gas from the ammonia absorber has had most of its naphthalene content removed with the tars during the primary cooling of the gas, a significant amount of naphthalene vapor remains in the coke oven gas at this point. Further cooling of the gas is achieved in a final cooler to lower the temperature of the gas stream for efficient processing in a light oil by-product recovery stage.
The final cooling is usually accomplished by the direct contact of the gas with a cooling liquid. The gas may be passed through a spray of the cooling liquid or it may be bubbled through the liquid. The cooling liquid may be water or it may be a solvent for naphthalenic materials such as washing oils or benzol oils from the light oil recovery plant.
Water cooling of the coke oven gas in the final coolers from about 45.degree.-60.degree. C. to about 20.degree.-30.degree. C. results in the naphthalene vapors precipitating as crystallized solids. In addition, a quantity of water vapor also condenses from the coke oven gas, which condensate is contaminated with dissolved ammonia, cyanides, sulfides and phenols in dilute concentrations.
Where a solvent for naphthalene is used to cool the gas, the precipitated naphthalenic solids are immediately dissolved in the cooling liquid. In a solvent cooling process, the liquid effluent from the final cooler must be separated by use of a decanter into a condensed water stream and a naphthalene enriched solvent stream from which the dissolved naphthalene must be removed as it accumulates to permit cooling and recycling of the cooling liquid.
It is known in the art to "bleed off" a portion of the solvent stream emanating the decanter and to remove the dissolved naphthalene from this bleed stream of solvent in a naphthalene stripper or in a combination naphthalene stripper-wash oil still. However, during the operation of a recirculating wash oil final cooling system for coke oven gas having a wash oil-bleed stream scheme, several problems developed which adversely affected the efficiency and performance of the process equipment.
The wash solvent stream emerging from the decanters, which are designed to separate the condensed water from the wash solvent, nevertheless contained relatively high water concentrations of above 0.5% (wt.). It was discovered that the high water concentrations in the wash solvent stream resulted from the poor separation of the water phase and wash solvent phase in the decanters because a stable emulsion ("muck") of naphthalene enriched wash solvent, water and solids had formed. This "muck" is actually a mixture of solids and two liquids which are immiscible in each other and are of different specific gravities.
The decanter is designed for the lighter wash solvent to overflow and for the heavier water to discharge from the bottom. While most of the "muck" distributes itself between the wash solvent and water phases, a not insubstantial amount does manage to overflow with the wash solvent into the recycle loop and eventually into the bleed stream to the stripper.
Therefore, the naphthalene stripper-wash oil still did not receive a "dry" bleedstream of wash solvent which was less than 0.5% (wt.) water and was substantially free of solids, but instead was fed a wash solvent having a 1.0-4.5% (wt.) water and a 5-300 ppm solids content.
Unfortunately, the "muck" also continually accumulates to the extent that it could eventually fill the decanter. One solution is to continually discharge the "muck" to waste; however, this results in a constant loss of wash solvent.
The presence of this high concentration of water and solids severely limited the ability of the stripping steam passed through the naphthalene stripper-wash oil still to remove naphthalene from the wash solvent. When injected into the top of the stripper at the designed operating temperature of about 115.degree. C., the water-containing wash solvent lowered the stripping temperature to about 101.degree. C. because the steam was being used to volatilize the water instead of stripping the naphthalene. Consequently, the naphthalene content of the wash solvent leaving the naphthalene stripper increased to an average of about 5% (wt.), much higher than the required 1.7% necessary for proper naphthalene absorption in the final cooler. The entire coke oven gas final cooling-naphthalene removal process had been designed to afford a stripped wash solvent having about 1.7% (wt.) naphthalene from the stripper-still unit for use as make-up wash solvent that would assure the required removal of naphthalene from the coke oven gas in the final cooler. If naphthalene is not removed sufficiently to yield a stripped wash solvent meeting the design criterion, the naphthalene concentration in the recycling wash solvent will increase to a level which is in equilibrium with the naphthalene concentration in the coke oven gas. Consequently the amount of naphthalene removed from the coke oven gas in the final cooler will be reduced causing naphthalene-fouling problems downstream in the coke oven gas by-products process equipment such as the light oil scrubber, the gas mains and the equipment for underfiring the coke oven batteries.
The water-containing wash solvent prior to its injection into the stripper was heated to a temperature above 125.degree. C. in an attempt to facilitate the evaporation of the water and permit the stripping steam to strip the naphthalene from the wash oil to the degree for which the system was engineered. This did not solve the problem. Moreover, in addition to the naphthalene content of the stripper wash solvent remaining at about 5% (wt.) and the stripping temperature remaining at about 101.degree. C., there developed severe frothing on the top trays of the stripper and carry-over into the vapor-to-oil heat exchanger which follows the stripper. In order to remove naphthalene from the wash solvent to the level for which the stripper-still was designed, an increased amount of stripping steam would be required under such conditions. It would also necessitate extensive modifications to the stripping still and additional operating costs. Furthermore, the high concentration of naphthalene and the presence of solids in the wash solvent upon recycling to the final coolers contribute to the formation of the stable "muck" emulsion layer in the wash solvent decanters. In sum, the stripper-still unit and the decanters were not performing as expected.
There is a need to continually provide the naphthalene stripper with a clean, dry wash solvent bleed stream which contains less than 0.5% (wt.) water.
There is a need to maintain a low concentration of naphthalene in the stripper wash solvent stream for recycling as make-up wash solvent.
Further there is a need to minimize the amount of wash solvent that must be removed, or "blown-down", from the final cooling-naphthalene removal system and to reduce the subsequent addition of fresh wash solvent that must be added to the system.
There is also a need to control the formation of a water and wash solvent emulsion in the decanters of a recirculating wash solvent final cooling system.
There is a further need to improve the performance of the naphthalene removal equipment without increasing the usage of stripping steam or making extensive modifications to existing facilities.
There is a still further need to prevent uncontrolled frothing on the top trays of the naphthalene stripper.
There is also a need to provide uninterrupted liquid-liquid-solid separation capabilities that can remove the "muck" from the wash solvent bleed stream in order to maintain the final cooling-naphthalene removal system as a continuous process rather than a bulk process.